1. Field of the Invention
The present invention relates generally to bicycles. More particularly, the present invention relates to a bicycle frame including a rear wheel suspension assembly suitable for use in connection with off-road bicycles.
2. Description of the Related Art
Off-road bicycles, or mountain bikes, may be equipped with front and rear suspension assemblies operably positioned between the front and rear wheels, respectively, and the frame of the bicycle. Providing front and rear suspension on a mountain bike potentially improves handling and performance by absorbing bumps, and other rough trail conditions, which may be encountered while riding off-road. As the sport of mountain biking has evolved, the size and difficulty of the obstacles that have become commonplace has increased in scale. As a result, bicycle manufacturers have attempted to continually increase the amount of suspension travel, or the distance that the front and rear wheels may move relative to the frame, of their respective all-purpose mountain bikes.
For comfort, performance and handling reasons, it is desirable to limit the size of a bicycle frame in both longitudinal (lengthwise) and lateral directions. As a result, only a limited amount of space is available before the bicycle becomes too long or too wide. As the amount of suspension travel increases, it becomes more difficult to achieve the performance objectives of the suspension assembly (e.g., isolating pedal and brake forces, providing a desired leverage rate or rate progression, etc.) while also accommodating necessary or desired features or components of the bicycle (e.g., a movable front derailleur, water bottle cages, etc.) within the available space.
One common bicycle rear suspension design involves a single lever, or swingarm, supporting a rear wheel at one end and being pivotally connected to the bicycle frame at the other end. Although such a system is simple and reliable, the single lever, or single pivot, rear suspension design suffers from a relatively large amount of pedal forces and braking forces being transmitted into the rear suspension assembly. Furthermore, in order to provide a large amount of suspension travel, it is necessary to increase the length of the swingarm. Practical constraints, such as limiting the overall length of the bicycle, necessitates placing the pivot for the single lever in a position forward of the pedal crank assembly of the bicycle. As a result, the swingarm typically passes above the pedal crank axis, through a location where the front derailleur would normally be placed. As a result, many single pivot, long travel rear suspension bicycles are prevented from utilizing a front derailleur, which severely limits the number of gear ratios available to the rider.
Rear suspension designs that include multiple lever members and, therefore, multiple pivots, typically exhibit better isolation of pedaling forces and braking forces from the rear suspension. Typically, a multiple lever rear suspension assembly will have a pair of lower arms (i.e., chain stays) pivotally connected to the bicycle frame at a forward end and a link member pivotally mounted to the main frame at a location above the chain stays. A pair of rearward arms (i.e., seat stays) are pivotally connected between rearward ends of the chain stays and link member. The rear wheel may be carried by either of the chain stays or seat stays. Typically, the rear shock absorber is operably positioned between the link member and the main frame. As a result, placement of the shock absorber is at a relatively high position within the bicycle frame, thereby raising the overall center of gravity of the bicycle, which is detrimental to the handling qualities of the bicycle. This drawback is magnified in large suspension travel designs, wherein the size of the rear shock absorber is typically increased.
One proposed solution is to extend a forward end of the link member beyond the seat tube of the main frame and mount the shock absorber in a generally vertical arrangement in front of the seat tube. However, such a design necessitates the use of a long link member, which then must also be increased in size in order to resist bending or torsional forces. The large link member offsets a substantial portion of the lowered center of gravity benefit of the lowered shock position. In addition, the elongated link member increases the lever arm of forces introduced into the suspension assembly through the rear wheel. As a result, a large amount of force is applied to the main frame by the lever, which necessitates enlarging and/or reinforcing the main frame to handle these stresses. In addition, the placement of the shock absorber adjacent the seat tube typically precludes the use of a front derailleur.